Decoupling of high-pressure H2 production from serpentinization and magnetite in subduction zones

Serpentinization plays a central role in geological, geochemical, and microbiological processes at various depths and conditions. While mineralogical and geochemical patterns of serpentinization are known at low-pressure and temperature, equivalent processes taking place at much greater depths and elevated temperatures in subduction zones are less constrained. Here we present the results of thermodynamic calculations simulating chemically complex aqueous alteration of ultramafic rocks relevant to three evolutionary stages of subduction, from infancy to maturity, and for three different fluid sources, namely metabasite, metasediment, and serpentinite. We found that magnetite production and H ₂ and CH ₄ concentrations are decoupled from serpentinization at these conditions, and strongly dependent upon minimal peridotite compositional variations. This has major implications on the interpretation of geophysical investigations of hydrated mantle wedge domains. Carbon isotopes exhibit large variations (> 10‰) for minimal changing compositions in some cases, with important implications on the isotopic diversity of high-temperature abiotic CH ₄ .